clear
Rz=6400000
H=19100000
R=Rz+H
T=12*3600
dT=T/1000
t=0:dT:T
phi=(t*2*pi)/T
x=R*cos(phi)
y=R*sin(phi)
vx=diff(x)/dT;
vy=diff(y)/dT;
vvvv=[vx;vy];
v=sqrt(vx.^2+vy.^2);
plot(v);
Rsv=[x;y];
Rv=Rsv-Rz;
%Vv=-v.*Rv/norm(Rv);

for i=1:1000
    Vv(i)=-dot(vvvv(1:2,i),Rv(1:2,i))/norm(Rv);
end